For many years a trend in the microelectronics industry has been to place an increasing number of microelectronic devices in an increasingly smaller area. As a result, the components of the microelectronics devices must themselves decrease in size and spacing. One component of microelectronic devices that is under such size pressure is the array of solder bumps that is used in mounting techniques such as flip chip or controlled collapse chip connect (C4) technology. Conventional stencil mask printing technology that is currently in wide use for such substrate solder bumping, when used for high density interconnect (HDI) substrates, causes significant bump height variation, bump bridging, and low volume and missing solder bump issues, which often result in severe problems during chip attachment. As an example, local mask deformation can lead to solder paste bleeding between solder resist openings (SROs), resulting in bridged bumps. Another common problem is that upon mask lift-off, a considerable amount of solder paste adhering to the walls of the mask apertures is taken away, which also leads to severe bump height variation, particularly for smaller SRO sizes. Next generation HDI substrates are projected to have a minimum pitch of 150 micrometers and lower, so stencil printing will likely only become more problematic. In fact, poor yields caused by the foregoing and other problems have been observed even at pitches of 150 micrometers.
The foregoing and other issues have prompted attempts to develop alternative substrate solder bumping technologies. One such technology deals with electrokinetic particle deposition in a fluidized bed, a technique originally developed for wafer bumping. Electrokinetic deposition, however, requires that the deposition surface be electrically conductive, a requirement that is inherently satisfied for wafers but not for substrates. Accordingly, there exists a need for a method of enabling electrokinetic solder deposition for fine pitch substrate bumping.
For simplicity and clarity of illustration, the drawing figures illustrate the general manner of construction, and descriptions and details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the discussion of the described embodiments of the invention. Additionally, elements in the drawing figures are not necessarily drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of embodiments of the present invention. The same reference numerals in different figures denote the same elements.
The terms “first,” “second,” “third,” “fourth,” and the like in the description and in the claims, if any, are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in sequences other than those illustrated or otherwise described herein. Furthermore, the terms “comprise,” “include,” “have,” and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to those elements, but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.
The terms “left,” “right,” “front,” “back,” “top,” “bottom,” “over,” “under,” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein. The term “coupled,” as used herein, is defined as directly or indirectly connected in an electrical or non-electrical manner.